This invention relates generally to a memory system for recording and reproducing digital information using a high intensity energy source, such as a laser, to focus energy on a thermoplastic surface, and, in particular, an apparatus and a method for reproducing the data by using a lower intensity laser beam to sense the topography of the surface.
Laser apparatus for both reading and writing data on a moving surface is generally known, and various features of such systems are disclosed, in particular, in U.S. Pat. Nos. 3,262,122 to H. Fleischer et al., 3,475,760 to C. O. Carlson, 3,815,146 to L. A. Nash and 3,825,323 to D. C. Landwer. Collectively these patents describe the operation of a laser read/write arrangement and all are incorporated by reference herein.
In the laser writing process the laser beam is focused on the surface of a moving film and turned on and off to "write" by forming a series of minute melted depressions in the surface. A single depression corresponds to a digital bit which can be read out in various ways, such as by measuring the reflectivity the surface provides when illuminated by a low power beam. This process is capable of recording vast amounts of information per unit area of the film, because of the small size of the focused beam which can be used; for example, in one recent application of the process, over 10,000,000 individual bits of binary data have been recorded per square inch of recording surface. In these laser recording processes, the laser beam is moved relative to the surface in a regular pattern of one type or another to form a track, and the laser is effectively turned "on" or "off" according to the binary data to record 1's and 0's along the track by melting or not melting in accordance with the data.
In these systems it is advantageous to use a rotating disc or record as the recording member, with a movable recording arm positioning the laser beam such that the recording process generates a spiral track or a set of concentric circular tracks in the record surface, generally analagous to the process used in making phonograph records. The rotating disc approach is very attractive in that the recording process is simple, large amounts of information can be recorded in a relatively small area, and playback or read-out is relatively simple.
While the extremely high density of information melted into the film is desirable, this high density creates problems in reproduction of the data. The focused spot of the laser beam used for reading must be precisely positioned so that it is superimposed upon the desired data grooves of one selected circular track (or portion of the spiral track). These tracks are very narrow and are closely spaced to one another; hence, the drive mechanism which positions the read spot must be held to very high tolerances to insure the repeated and accurate location of the read spot.
It is an object of the present invention to improve the reading technique and to reduce, if not eliminate, the need for these extremely high mechanical tolerances while still insuring that the read spot will impinge only upon the desired track without being erroneously positioned on the space between tracks or on an adjacent track.